http://www.oss.net/dynamaster/file_archive/100102/0a947a77d7...

In one part, it says:

The greatest material curse to the profession, despite all its advantages, is undoubtably the telephone. It is a constant source of temptation to slackness. And even if you do not use it carelessly yourself, the other fellow, very often will, so in any case, warn him. Always act on the principle that every conversation is listened to, that a call may always give the enemy a line. Naturally, always unplug during confidential conversations. Even better is it to have no phone in your room, or else have it in a box or cupboard.

That's talking about the plane old telephone in the 1960s.

The message I get from it? If your life is on the line, consider not using any electronic communication at all.